1. Field of the Invention
This invention relates to a charging roller, and more specifically to a charging roller used in a contact charging device that applies a voltage to a conductive member that is in contact with an electrophotographic photosensitive member to charge the electrophotographic photosensitive member surface to a predetermined electric potential.
2. Related Background Art
Although many methods have been proposed as an electrophotographic method, the generally used method has processes in which an electric latent image is formed on a photosensitive member through various means by using a photoconductive substance, and after the latent image have been developed by using toner to form a visible image and the toner image has been transferred onto a transfer medium such as paper on demand, the toner image is fixed on the transfer medium through heat, pressure or the like to obtain a copied object. Here, toner particles remaining on the photosensitive member without being transferred on the transfer medium is removed from the photosensitive member through a cleaning process.
Conventionally, the corona charger was used as a charging device for electrophotography. In recent years, instead of this, a contact charging device has been put into practical use. This device aims at low ozone and low power consumption, and among those devices of this type, in particular, a roller charging system in which a conductive roller is used as its charging member has been desirably used from the viewpoint of charging stability.
In the roller charging system, a conductive elastic roller is made in press-contact with a member to be charged so that a voltage is applied to this member; thus a charging process to the member to be charged is carried out.
More specifically, since the charging process is carried out by discharging to the member to be charged from the charging member, the charging process is started by applying a voltage that exceeds a certain threshold value. For example, when the charging roller is brought into pressure contact with an organic photosensitive member (OPC photosensitive member) having a photoconductive layer with a thickness of 25 μm, upon application of a voltage of about 640 V or more in terms of the absolute value, the surface electric potential starts to rise, and thereafter, the photosensitive member surface electric potential increases linearly with a gradient of 1 with respect to the applied voltage. Hereinafter, this threshold voltage value is defined as a charge starting voltage Vth.
In other words, in order to obtain the photosensitive member surface potential Vd required for electrophotography, the charging roller needs a DC voltage, Vd+Vth, that is more than a voltage required for the image forming process itself. This method which applies only the DC voltage to the contact charging member to carry out a charging process is referred to as DC charging.
However, in the DC charging, since the resistance value of a contact charging member tends to vary due to environmental variations and the like, or since Vth fluctuates when the thickness varies due to abrasion of the photosensitive member, with the result that it becomes difficult to set the electric potential of the photosensitive member to a desired value.
For this reason, in order to further attain uniformity in charging, an AC+DC charging system, which applies a voltage generated by superposing an AC component having a peak-to-peak voltage of more than 2×Vth on a DC voltage corresponding to desired Vd to the contact charging member, is used. This system, which aims at an equalizing effect of electric potential by the use of AC, converges the electric potential of the member to be charged to Vd that comes to be the center of the peak of the AC voltage so that the electric potential is hardly influenced by disturbances in the environment or the like.
With respect to the conductive member used for charging, for example, a member in which a surface layer is formed on a conductive support member by using a conductive seamless tube has been proposed (see the specification of U.S. Pat. No. 4,967,231). Moreover, a seamless tube made of fluororesin (see Japanese Patent Application Laid-Open No. H05-2313) and a multi-layer tube constituted by layers having different conductive properties (see Japanese Patent Application Laid-Open No. H05-96648) have been disclosed. With respect to the method for manufacturing the charging member, the above-mentioned conventional techniques include a method in which the charging member is formed by using an insertion process. Moreover, a surface formation method using a cross-head extruder has been proposed (see Japanese Patent Application Laid-Open No. H06-58325).
In such a method in which the charging roller is formed by using a seamless tube, even when a foaming material is used as an elastic body layer on a base member, the foaming material is further coated with the seamless tube so that a smooth face can be formed to provide a more uniform charging process.
With respect to the method for allowing the seamless tube to have a conductive property, in general, an ion-conduction method in which a salt is used as a conductive agent and an electron conduction method in which a material such as carbon black, a conductive metal oxide and metal powder is used as a conductive agent have been proposed. When the conductive property is imparted by ion conduction, problems are raised in that the resistance value tends to fluctuate greatly due to the environment and the salt tends to contaminate the photosensitive member since the salt comes into contact with the electrophotographic photosensitive member. Therefore, in the present invention, carbon black was used as the conductive agent.
However, when the seamless tube which uses a thermoplastic resin as the base material is produced, resistance changes due to the tube expansion and contraction caused by the size of the foaming layer forming the base layer upon coating the tube. Another problem is raised in that the tube is expanded or contracted and resistance changes due to the influence of the foaming layer forming the base layer involved in changes in the environment or a lapse of time.